How to speed up PHP powered websites? [closed]

Question

Many people are talking about somethings like accelerator, opcode, etc to speed up a website's performance. To be honest I do not know how to make any PHP website perform well and when do I need to use such things and what if I use them will that cost me a lot? I generally like websites to perform well with excellent and quick load time for the user (I know the load time has other factors such as the internet speed but what I care about here is the performance of the PHP code itself how to make it perform better).

So, could you tell me about all the ways to speed up any PHP website and whether they are possible and easy to implement or not?